Anatomical relationships between Sylvian fissure and the sphenoid ridge

During pterional craniotomy, the sphenoid ridge is resected to facilitate the dissection of the arachnoid in the Sylvian fissure. It is a common intra-operative perception that a deep sphenoid ridge often accompanies a shallow fissure. We explored this unique relationship between the sphenoid ridge and the Sylvian fissure. We studied 46 fresh adult cadavers and measured the depth of both the sphenoid ridges and the Sylvian fissure. The deepest portion of the sphenoid ridge was determined and its depth measured by using a narrow ruler with a sliding gauge. The depth of the Sylvian fissure was measured from the middle cerebral artery bifurcation perpendicularly to the surface of the temporal pole where the arachnoid was cut. An inverse relationship exists between the Sylvian fissure and the sphenoid ridge. The depth of the sphenoid ridge averages 7.28 mm +/- 0.3 mm while the depth of the Sylvian fissure averages 9.0 mm +/- 0.4 mm. Approximately 25% of studied population have deep ridges, i.e. greater than 10 mm, and approximately 25% have shallow ridges, i.e. less than 4 mm. The extent to which the sphenoid ridge projects into the Sylvian fissure determines the depth of the Sylvian fissure thus explaining their inverse relationship. Importantly, encountering a shallow sphenoid ridge during surgery most likely will result in the dissection of a deep Sylvian fissure while a deep ridge predicts a shallow fissure.